A series of exceptionally dry summers that caused famine and social breakdown were behind one of the most severe threats to Roman rule of Britain, according to new academic research.
The rebellion, known as the “barbarian conspiracy”, was a pivotal moment in Roman Britain. Picts, Scotti and Saxons took advantage of Britain’s descent into anarchy to inflict crushing blows on weakened Roman defences in the spring and summer of AD367.
Senior Roman commanders were captured or killed, and some soldiers reportedly deserted and joined the invaders. It took two years for generals dispatched by Valentinian I, emperor of the western half of the Roman empire, to restore order. The last remnants of official Roman administration left Britain about 40 years later.
Warning of the possible consequences of drought today, Tatiana Bebchuk, a researcher at Cambridge’s department of geography, said: “The relationship between climate and conflict is becoming increasingly clear in our own time, so these findings aren’t just important for historians. Extreme climate conditions lead to hunger, which can lead to societal challenges, which eventually lead to outright conflict.”
The study, published in Climatic Change, used oak tree-ring records to reconstruct temperature and precipitation levels in southern Britain during and after the barbarian conspiracy. Combined with surviving Roman accounts, the data led the authors to conclude that severe summer droughts were a driving force.
Little archaeological evidence for the rebellion existed, and written accounts from the period were limited, said Charles Norman of Cambridge’s department of geography. “But our findings provide an explanation for the catalyst of this major event.”
Southern Britain experienced an exceptional sequence of remarkably dry summers from AD364 to 366, the researchers found. In the period AD350-500, average monthly reconstructed rainfall in the main growing season was 51mm. But in AD364, it fell to 29mm. AD365 was even worse with 28mm, and the rainfall the following year was still below average at 37mm.
Prof Ulf Büntgen of Cambridge’s department of geography said: “Three consecutive droughts would have had a devastating impact on the productivity of Roman Britain’s most important agricultural region. As Roman writers tell us, this resulted in food shortages with all of the destabilising societal effects this brings.”
The researchers identified no other major droughts in southern Britain in the period AD350-500 and found that other parts of north-west Europe escaped these conditions.
By AD367, the population of Britain was in the “utmost conditions of famine”, according to Ammianus Marcellinus, a soldier and historian.
Norman said the poor harvests would have “reduced the grain supply to Hadrian’s Wall, providing a plausible motive for the rebellion there, which allowed the Picts into northern Britain”.
The study suggested that grain deficits may have contributed to other desertions in this period, and therefore a general weakening of the Roman army in Britain.
Military and societal breakdown provided an ideal opportunity for peripheral tribes, including the Picts, Scotti and Saxons, to invade the province.
Andreas Rzepecki, from the Rhineland-Palatinate General Directorate for Cultural Heritage in Trier, said: “The prolonged and extreme drought seems to have occurred during a particularly poor period for Roman Britain, in which food and military resources were being stripped for the Rhine frontier.
“These factors limited resilience, and meant a drought-induced, partial-military rebellion and subsequent external invasion were able to overwhelm the weakened defences.”
The researchers expanded their climate-conflict analysis to the entire Roman empire for the period AD350-476. They reconstructed the climate conditions immediately before and after 106 battles and found that a statistically significant number of battles were fought following dry years.
